1. Field of the Invention
The present invention relates to an intelligent assisted transmission for a wireless communication system containing a Relay Station (RS). More particularly, the present invention relates to an intelligent assisted transmission method for a Multicast Broadcast Service (MBS) in a wireless communication system.
2. Description of the Related Art
In a conventional Institute of Electrical and Electronics Engineers (IEEE) 802.16 system, an MBS is transmitted in multicast/broadcast areas. A multicast/broadcast area includes one or several base stations. These base stations adopt the same modulation and coding method when transmitting the MBS and transmit the same content with the same frequency. In a middle part of an MBS area, the same signals received from all adjacent base stations are superimposed with a high signal to interference noise ratio. In edges of the MBS area, a user equipment receives both the multicast/broadcast signal transmitted by the base stations in the MBS area and other signals transmitted by base stations from non-MBS areas with a low signal to interference noise ratio.
The IEEE 802.16m system prescribes that a multi-base station MBS, which supports macro diversity, should have a spectrum efficiency of 2 bps/Hz (when the base station interval is less than 1.5 Km) or 4 bps/Hz (when base station interval is less than 0.5 Km). In a center of an MBS area, since all signals received by the user equipment are available signals rather than interference, it is not difficult to reach such spectrum efficiency. However, at edges of the MBS area, due to interference from base stations from non-MBS areas, the attainable spectrum efficiency is very low. Simulation for Long Term Evolution (LTE) shows that spectrum efficiency at cell edges is usually lower than 0.1 bps/Hz.
The above analysis shows that there are large spectrum efficiency differences between the edges and the center of an MBS area. Because the whole MBS area adopts the same modulation and coding method, a modulation and coding method which meets the requirements of the IEEE 802.16m system cannot perform normal decoding at the MBS area boundary.
Hierarchical modulation has been proposed to meet required channel quality differences between the various user equipment. In the hierarchical modulation, a user equipment with poor channel quality only needs to make demodulation (such as QPSK) at the basic layer to get preliminary information (such as low-quality video pictures), while a user equipment with high channel quality can perform demodulation (such as 16QAM or 64QAM) at a higher layer to get further information (such as high-quality video pictures). However, a normal rate difference between the basic layer and the higher layers in hierarchical modulation is 1:2 or 1:3. For the edges and center of the MBS area, a rate difference of around 1:20 cannot be reached.
In LTE, Huawei has suggested to adopt a method of dynamic multicast service to solve the MBS area boundary problem. Specifically, the system tracks down a movement of the user equipment between cells based on a counting process, dynamically adjusts the size of the MBS area, and thus makes macro diversity always exist at a position in which the user equipment is located. However, in the IEEE 802.16m system, a user equipment under an idle mode can cross cell boundaries without informing the base station. Consequently, it is not possible to use such a method in the IEEE 802.16m system to solve the MBS area boundary problem.